A modification of the rat isolated seminal vesicle preparation is described, emphasizing the necessity to use younger animals (40-50 days old and weighing between 125 and 150 g) and to expel thoroughly all vesicular contents. Under the experimental conditions used (tissues suspended under a resting tension of 350 mg in a continuous flow of a modified Krebs solution run at the rate of 15 ml/min, maintained at 32 degrees C, and bubbled with 5% CO2 in O2), the preparation was quite sensitive, but only to a few selected agonists, and remained viable for over 4-6 hr. Adrenaline, noradrenaline, dopamine, and acetylcholine all produced concentration-dependent and reproducible contractions. However, histaminergic, serotoninergic, purinergic, and opioid agonists were inactive as were prostaglandins of the E and F series and the polypeptides angiotensin, vasopressin, and oxytocin. In general, the tissue was rather insensitive to relaxant drugs, with only papaverine and sodium nitrite producing some relaxation in tissues previously contracted by carbachol. Advantages of the preparation include marked responsiveness, but only to a few selected agonists, and suitability for use as a paired tissue. It is suggested that employed under suitable experimental conditions, the preparation deserves a more frequent consideration for use during pharmacological investigations concerned with postsynaptic aspects of noradrenergic or cholinergic transmission.